Quantitative assessment of various low-GWP zeotropic mixtures of CO2 as the working fluid in two-phase natural circulation loops

This paper represents the first quantitative investigation on how the use of zeotropic mixtures in a two-phase natural circulation loop (NCL) with carbon dioxide (CO2) as the base refrigerant (CO2 being attractive for its low pressure drop and high heat transfer rate) can address the issues of high standstill and operating pressures associated with the use of pure CO2, and yet increases the pressure drop only by a small amount and improves upon the global warming potential (GWP). Three refrigerants are considered as the second component in the zeotropic mixture - dimethyl ether (DME, CH3OCH3, R-E170), propane (C3H8, R290) and 2,3,3,3-Tetrafluoropropene (CH2=CFCF3, R1234yf) which is a hydrofluoroolefin (HFO). A complete mathematical theory is formulated for the determination of the performance of an NCL working with zeotropic mixtures, using the homogeneous two-phase flow approach, and appropriate correlations for pressure drop and heat transfer coefficients. The relative performances of the NCL using various zeotropic mixtures are quantitatively assessed. The present study reveals that no single zeotrpic mixture excels in terms of all of the four criteria adopted and provides a clear guideline to the designer in the choice of the zeotropic mixture depending on the functional requirement. The CO2- R1234yf mixture gives the lowest entropy generation rate and the lowest temperature difference between the source and the sink. The CO2- DME mixture produces the minimum operating and standstill pressures. The CO2- propane mixture turns out to be a good compromise when all four criteria are considered important.